1. Field of the Invention
The present invention relates to a boosting circuit of charge pump type and a boosting method for boosting an input voltage to a voltage three or higher integer times.
2. Description of the Related Background Art
Liquid crystal displays need a voltage higher than a power supply voltage to drive a liquid crystal display panel. For reduction in size and weight, a boosting circuit of charge pump type for boosting the power supply voltage is incorporated into a semiconductor integrated circuit that comprises a drive circuit (see Japanese Patent Application Laid-Open No. 2004-64937).
FIG. 1 shows the configuration of a conventional triple boosting circuit of charge pump type. The conventional boosting circuit has switch elements SW1A, SW1B, SW2A, SW2B, SW3A, and SW3B which are included in a semiconductor integrated circuit 1. The switch elements SW1A, SW1B, SW2A, SW2B, SW3A, and SW3B are on-off switches which are turned on/off by a not-shown controller. The semiconductor integrated circuit 1 has external component connection terminals A1 to A3, AC+, AC−, and AG. The switch elements SW1A and SW2B are each connected at one end to the connection terminal A1. The switch elements SW2A and SW3B are each connected at one end to the connection terminal A2. The switch element SW3A is connected at one end to the connection terminal A3. The switch element SW1B is connected at one end to the connection terminal AG, the ground terminal. The other ends of the switch elements SW1A, SW2A, and SW3A are connected to the connection terminal AC+. The other ends of the switch elements SW1B, SW2B, and SW3B are connected to the connection terminal AC−. The boosting circuit also includes capacitors Ca, C1, C2, and C3 for charge accumulation, which are external components of the semiconductor integrated circuit 1. One end of the pumping capacitor Ca is connected to the connection terminal AC+. The other end is connected to AC−. One end of the capacitor C1 is connected to the connection terminal A1. One end of the capacitor C2 is connected to the connection terminal A2. One end of the capacitor C3 is connected to the connection terminal A3. The other ends of the capacitors C1, C2, and C3 are connected to the connection terminal AG and are grounded (connected to Vss). For ease of description, the ground potential Vss will hereinafter be assumed to be 0 V.
In the conventional triple boosting circuit, an input voltage is applied to the capacitor C1. The input voltage will be referred to as VL1. In a boosting operation, the operations of first to fourth steps are repeated. The first to fourth steps have the same duration. As shown in FIG. 2, in the initial first step, the switch elements SW1A and SW1B are turned on, and the switch elements SW2A, SW2B, SW3A and SW3B are turned off. In the next second step, the switch elements SW1A, SW1B, SW3A, and SW3B are turned off, and the switch elements SW2A and SW2B are turned on. In the third step, the switch elements SW1A and SW1B are turned on, and the switch elements SW2A, SW2B, SW3A and SW3B are turned off. In the fourth step, the switch elements SW1A, SW1B, SW2A, and SW2B are turned off, and the switch elements SW3A and SW3B are turned on.
In the first step, the turning-on of the switch elements SW1A and SW1B applies the input voltage VL1 to the capacitor Ca, whereby the capacitor Ca is charged up. The voltage C+ on the connection terminal AC+ of the capacitor Ca becomes VL1, and the voltage C− on the connection terminal AC− becomes Vss.
In the second step, the turning-on of the switch elements SW2A and SW2B applies the input voltage VL1 plus the voltage VL1 of the capacitor Ca to the capacitor C2, whereby the capacitor C2 is charged up. The voltage C+ of the capacitor Ca becomes VL1+VL1, and the voltage C− on the connection terminal AC− becomes VL1. The voltage VL2 on the connection terminal A2 of the capacitor C2 becomes VL1+VL1.
In the third step, the turning-on of the switch elements SW1A and SW1B applies the input voltage VL1 to the capacitor Ca, whereby the capacitor Ca is charged up. The voltage C+ on the connection terminal AC+ of the capacitor Ca becomes VL1, and the voltage C− on the connection terminal AC− becomes Vss.
In the fourth step, the turning-on of the switch elements SW3A and SW3B applies the voltage VL2 of the capacitor C2 plus the voltage VL1 of the capacitor Ca to the capacitor C3, whereby the capacitor C3 is charged up. The voltage C+ of the capacitor Ca becomes VL2+VL1, and the voltage C− on the connection terminal AC− becomes equal to VL2 of the capacitor C2. Consequently, the voltage VL3 on the connection terminal A3 of the capacitor C3 becomes VL2+VL1, i.e., 3VL1.
The operations of the first to fourth steps are repeated in succession, whereby the voltage VL3 of the connection terminal A3 is maintained at the triple boost voltage 3VL1, and the voltage VL2 of the connection terminal A2 at the double boost voltage 2VL1.
When the conventional boosting circuit starts a boosting operation, relatively high currents flow into the uncharged capacitors C2 and C3 transiently, producing peak currents. The input voltage VL1 of the conventional boosting circuit is used as the power supply voltage of other circuits. For example, when the boosting circuit is used as the drive voltage generating circuit of an STN liquid crystal display chip, the input voltage VL1 is used as the power supply voltage of other circuits like logic circuits and an oscillator circuit in the semiconductor chip. However, in such a semiconductor chip, the input terminal of the power supply voltage is shared with those circuits for a reduction in chip size. There has thus been a problem in that the foregoing peak currents flowing into the capacitors C2 and C3 when the boosting circuit starts a boosting operation cause a temporary drop of the voltage VL1 to below the normal operating voltage in level, causing malfunction of the chip itself.